Rendered image showing grains
Changing the resolution of the image merely renders it bigger. In the image below, I've change it to 500%, and that would blow up both the X and Y dimensions five fold, i.e. making it 6400x3600 pixels rather than the 1280x720 entered in the X and Y boxes.  

From your screenshot, I'm guessing that you're using Cycles, and grain/noise in Cycles usually means too few samples. Try increasing the Render value under samples. What you need exactly depends highly on your lighting and materials among other things, so I can't say specificly what you'll need.  

Changing the Clamp values usually reduces noise as well, by clamping out very high intensities.  

You can read more about it here.